[Fix Request] Disable keytar in permanent settings

As discovered in https://github.com/flathub/com.visualstudio.code/issues/350#issuecomment-1672537287, the statement by [Microsoft in their documentation](https://code.visualstudio.com/docs/editor/settings-sync#_troubleshooting-keychain-issues) that keytar is disabled and the system secret store is used instead, is incorrect. In fact `disable-keytar` is set `false` still (at least in this Flatpak build).

All Settings Sync is broken due to keytar being enabled and interfering with the authentication setup halfway thru. The fix is to set `"disable-keytar": true` in the permanent runtime settings. This should be built into the Flatpak build.